1. Field of the Invention
The present invention relates to a photoelectric conversion device and image capturing device.
2. Description of the Related Art
These days, CMOS sensors have prevailed for digital cameras. One main reason is that the CMOS sensor can achieve a high S/N under ISO sensitivity conditions varying from ISO 100 to ISO 1600. A high-S/N CMOS sensor is effectively implemented using a column amplifier having a gain switching function. This is because random noise can be suppressed much more by switching the gain in a column amplifier of a narrow band (operation frequency of about several hundred kHz) than by applying the gain in a final output amplifier of a wide band (operation frequency of several MHz to several ten MHz). Generally when amplifying an analog signal, increasing the gain at as early a stage as possible is effective against random noise and fixed-pattern noise.
FIGS. 8 and 9 are circuit diagrams excerpted from Japanese Patent Laid-Open No. 6-339082. A photoelectric conversion device disclosed in Japanese Patent Laid-Open No. 6-339082 will be explained with reference to FIGS. 8 and 9. Outputs from BASIS type pixel units B1, B2, B3, and B4 are read out from an emitter signal line, voltage-amplified by column amplifiers A1, A2, A3, and A4, and then written in holding capacitors G1, G2, G3, and G4. Signals written in the holding capacitors G1 to G4 are read out in time series to a horizontal output line 4 in accordance with a control signal from a scanning circuit 1, and output outside via an output amplifier 3. The gains of the column amplifiers A1 to A4 are controlled by a power supply 2. The band of the column amplifiers A1 to A4 may be narrower than that of the output amplifier 3. The column amplifiers A1 to A4 can narrow the frequency band at which noise is integrated, compared to a case where the wide-band output amplifier 3 amplifies a voltage. Thus, the column amplifiers A1 to A4 can reduce random noise. Amplifying a voltage by the column amplifiers A1 to A4 is also effective to suppress fixed-pattern noise. Fixed-pattern noise is generated owing to relative variations between the capacitance values of the holding capacitors G1 to G4, variations between the parasitic capacitances of switches M41 to M44, and the like. Amplifying a voltage by the output amplifier 3 also amplifies such fixed-pattern noise. Thus, it is more advantageous to amplify a voltage by the column amplifiers A1 to A4.
As the number of pixels of a CMOS sensor increases, the read rate must be increased to obtain the same frame rate. For this purpose, the horizontal blanking period during which a signal is read from a pixel to a holding capacitor must be shortened. However, a solid-state image sensor in which the column amplifier switches the gain suffers the following problem in increasing the read rate.
Generally in an amplifier circuit, as the gain becomes higher, the band of the column amplifier becomes narrower. Gain switching changes the band of the column amplifier. Particularly when a high gain is set, the band narrows, degrading the response characteristic. To improve the response characteristic, the holding capacitor connected to the output of the column amplifier may be decreased to widen the band. However, a small holding capacitor leads to large relative variations between the capacitance values of holding capacitors, increasing fixed-pattern noise. For this reason, the lower limit of the capacitance value of the holding capacitor is defined by fixed-pattern noise, and its upper limit is defined by the response characteristic obtained when high sensitivity is set. However, as the read rate increases, these two requests cannot be satisfied simultaneously.